Twilio - Brings voice and messaging to your web and mobile applications.

Voxer - Turn your phone into a Walkie Talkie. Don't waste time on phone calls and voicemails.

Plivo - Plivo simplifies your customer engagement.

Sameroom - Chat interoperability-as-a-service

Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates

Investis - Investis helps companies to use digital channels to increase revenue, enhance employee engagement and influence corporate audiences.